Interplay between antiferrodistortive, ferroelectric, and superconducting instabilities inSr1−xCaxTiO3−δ

Abstract

$\phantom{\rule{4.pt}{0ex}}{\text{SrTiO}}_{3}$ undergoes a cubic-to-tetragonal phase transition at 105 K. This antiferrodistortive transition is believed to be in competition with incipient ferroelectricity. Substituting strontium by isovalent calcium induces a ferroelectric order. Introducing mobile electrons to the system by chemical nonisovalent doping, on the other hand, leads to the emergence of a dilute metal with a superconducting ground state. The link between superconductivity and the other two instabilities is a question gathering momentum in the context of a popular paradigm linking unconventional superconductors and quantum critical points. We present a set of specific-heat, neutron-scattering, dielectric-permittivity, and polarization measurements on $\phantom{\rule{4.pt}{0ex}}{\text{Sr}}_{1\ensuremath{-}x}\phantom{\rule{4.pt}{0ex}}{\text{Ca}}_{x}\phantom{\rule{4.pt}{0ex}}{\text{TiO}}_{3}$ $(0<x<0.009)$ and a study of low-temperature electric conductivity in $\phantom{\rule{4.pt}{0ex}}{\text{Sr}}_{0.9978}\phantom{\rule{4.pt}{0ex}}{\text{Ca}}_{0.0022}\phantom{\rule{4.pt}{0ex}}{\text{TiO}}_{3\ensuremath{-}\ensuremath{\delta}}$. Calcium substitution was found to enhance the transition temperature for both antiferrodistortive and ferroelectric transitions. Moreover, we find that $\phantom{\rule{4.pt}{0ex}}{\text{Sr}}_{0.9978}\phantom{\rule{4.pt}{0ex}}{\text{Ca}}_{0.0022}\phantom{\rule{4.pt}{0ex}}{\text{TiO}}_{3\ensuremath{-}\ensuremath{\delta}}$ has a superconducting ground state. The critical temperature in this rare case of a superconductor with a ferroelectric parent is slightly lower than in $\phantom{\rule{4.pt}{0ex}}{\text{SrTiO}}_{3\ensuremath{-}\ensuremath{\delta}}$ of comparable carrier concentration. A three-dimensional phase diagram for $\phantom{\rule{4.pt}{0ex}}{\text{Sr}}_{1\ensuremath{-}x}\phantom{\rule{4.pt}{0ex}}{\text{Ca}}_{x}\phantom{\rule{4.pt}{0ex}}{\text{TiO}}_{3\ensuremath{-}\ensuremath{\delta}}$ tracking the three transition temperatures as a function of $x$ and $\ensuremath{\delta}$ results from this study, in which ferroelectric and superconducting ground states are not immediate neighbors.